In many buildings, most particularly office buildings, the room ceiling on each floor is usually spaced below the structural floor panel of the next higher floor and is referred to as a drop ceiling. This spacing creates a return air plenum for the building's heating and cooling systems, which is usually continuous throughout the entire length and breadth of the floor.
If a fire occurs within a room or rooms on a floor and below the drop ceiling, it may be contained by the walls, ceiling, and floor of the room. On the other hand, if the fire reaches the plenum it can spread at an alarming rate, especially, if, as is often the case, flammable materials are located within the plenum. Inasmuch as the plenum is a convenient place to route wires and cables, both electrical power and communication types, unless these wires and cables are flame and smoke retardant they can contribute to the rapid spread of fire and smoke throughout the floor and, worse, throughout the building.
As a result of the potential danger presented by flammable insulation of wires and cables, the National Electric Code (NEC) prohibits the use of electrical cables in plenums unless they are enclosed in metal conduits. Such metal conduits are difficult to route in plenums congested with other items or apparatus, and where, for example, it is desirable or necessary to rearrange the office and its communications equipment, computers, and the like, the re-routing of the conduits can become prohibitively expensive. As a consequence, the NEC permits certain exceptions to the conduit requirement. Where, for example, a cable is both flame resistant and low smoke producing, the conduit requirement is waived provided that the cable, in tests, meets or exceeds the code's requirement for flame retardation and smoke suppression. Such tests must be conducted by a competent authority such as the Underwriters Laboratory.
In the prior art, data and other signal transmission has been carried out on cables in which the conductors are insulated with, for example, polyvinyl chloride (PVC). However, such cables too often result in transmission losses which are undesirably high for the transmission of high frequency signals. As a consequence, various alternative cable structures, using various types of materials, have been tried. A plenum cable having superior resistance to flame spread and smoke evolution is shown in U.S. Pat. No. 4,284,842 of Arroyo et al, which incorporates a metallic barrier sheath system which reflects radiant heat. For smaller size plenum cables, i.e., fewer than twenty-five pairs of conductors, such a structure is unduly expensive. In U.S. Pat. No. 5,162,609 of Adriaenssens et al there is shown a fire resistant cable in which the individual wires of the core have a dual insulation system comprising an inner layer of suitable plastic material and an outer layer of a flame retardant plastic material. The insulation system has the desirable characteristics of low dissipation factor and low dielectric constant, and the jacket which surrounds the core, which comprises flame retardant polyolefin material, also has low dissipation factor and dielectric constant. The dual insulation arrangement, however, represents an additional cost increment, especially for low pair cables, and can, in some cases, lead to increased structural return loss (SRL).
Certain standards have been established for cables used in buildings, such as the Commercial Building Telecommunications Cabling Standard TIA/EIA-568, in which cables are classified and categorized as to their electrical characteristics. Of the various categories, Category 5 is the highest rating and indicates a cable having stringent required maxima and/or minima for parameters of D.C. resistance, pair-to-ground capacitance, impedance, structural return loss (SRL), attenuation, and near end cross-talk. A Category 5 cable must meet or exceed these requirements and is the preferred cable in those applications where data transmission at high frequencies is necessary, which applies to most modem day office systems. In order for a Category 5 cable to be used as a plenum cable, it must meet the NEC requirements for flame and smoke retardation, i.e., it must pass the burn tests as used by, for example, the Underwriters Laboratory. Thus a Category 5 low pair count plenum cable must meet the standards for Category 5 and, also, the standards for flame and smoke retardation for plenum cables in which case it is a UL CMP plenum rated cable.
At the present time, almost all of the low pair, i.e., six or fewer, typically four twisted pairs, Category 5 cables that are commercially available use a tetra-flouoro ethylene/hexafluro propylene copolymer (FEP) as insulation for the individual wires forming the pairs, and a jacket of fluoropolymer material such as a copolymer of ethylene and chlorotrifluoroethylene (ECTFE). The FEP material most commonly used is Teflon.RTM. TE-4100, manufactured by DuPont, and an ECTFE material commonly used for the jacket is Halar.RTM. 985, supplied by Ausimont, U.S.A. When such materials are used in a low-pair cable it meets the performance requirements for Category 5 cable, provided that it has the required fire and smoke retardation for meeting the requirements for use as a plenum cable.
FEP materials, such as Teflon.RTM., are quite expensive and, at times, in limited or short supply, thereby making production of Category 5 plenum cable both expensive and limited as to quantity. In addition, Halar.RTM. 985, although excellent as to burn and smoke performance, is relatively stiff and often kinks, thereby making the cable somewhat difficult to route through any plenum and difficult to pull, and, the cable also is likely to be damaged when kinked.